802.11A does not have higher bandwidth than G. The price is to make a profit.
Not being able to get a HD stream from Netflix (>5Mbps) has nothing to do with 802.11G which is 54Mbps, it's the DSL that's the issue.
Not necessarily. While A and G both have the same theoretical max of 54Mbps, in practice, they achieve only about half that, and G falls behind in net throughput for a couple of reasons:
It uses the 2.4GHz band, which is also occupied by pretty much every other wireless device out there, including your XBox controllers, cordless phones, microwaves, and other 802.11b and g networks, significantly increasing the likelyhood of interference and dropped packets. (802.11a uses the 5GHz band, which is wider, and has almost no interference)
It generally coexists with a 802.11b stack on the same network device. If you connect an 802.11b device to a 802.11g hub, all your other 802.11g devices automatically drop down to a much lower rate, not much higher than the 802.11b device. (802.11a doesn't have this problem, since it doesn't coexist with any other standard)
Unless you have _only_ 802.11g devices on your network, and no other networks or devices in the area, 802.11a will be significantly faster and more stable. And note that the net throughput for 802.11b will generally be below 5Mbps (lots of error correction), which will almost certainly mess with netflix rates, as will dropped packets from interference, even with higher throughput.